1. Field of the Invention
This invention relates in general to optical rotation sensing devices. More particularly, this invention pertains to a method and apparatus for aligning the path of light beams transmitted about the optical cavity of a square ring laser gyroscope.
2. Description of the Prior Art
The conventional ring laser gyroscope with which the present invention is concerned includes planar top and bottom horizontal surfaces bordered by eight planar sides that form an octagon-shaped perimeter. Four non-adjacent sides form the mirror mounting surfaces. Each mounting surface should be substantially parallel to the opposed mounting surface and equidistant from the center of the frame to provide an optimum (maximum intensity) beam path. Due to unavoidable manufacturing tolerances of the fabrication process and inaccuracies introduced through polishing of the various surfaces, the perfect parallelism of the opposed mounting surfaces cannot be normally realized. Consequently, a beam path alignment process is required.
The planar alignment process is rendered difficult, if not impossible to achieve, due to the out-of-planeness of the square ring laser gyroscope. The "out-of-planeness" is herein defined as the non-planar deformation or tilt with respect to the horizontal surfaces of the gyroscope frame between two consecutive incident planes of the gyroscope. Such out-of-planeness is primarily induced by the imperfect parallelism of the opposed mirror mounting surfaces.
The combination of the out-of-planeness, which is caused by the non-planeness of the beam path, with certain unavoidable ambient magnetic fields possibly generated by nearby electromagnetic devices such as accelerometers can induce a magnetic bias sensitivity into the gyroscope output. For example, in a forty (40) centimeter ring laser gyroscope without the benefit of this invention, an arc-second of tilt angle produces output bias of between 0.01 to 0.04 degrees/hour/gauss. Such bias degrades the accuracy and thereby limits the gyroscope applications.
The out-of-planeness of the square ring laser gyroscope causes the cavity polarization state to be slightly elliptical (as opposed to linear), which in turn yields a magnetic bias sensitivity via the Faraday effect in the gain medium. The gyroscope magnetic bias sensitivity poses a great concern for high accuracy applications.
The cavity out-of-planeness can be varied by applying equal and opposite translations to two adjacent spherical mirrors. In principle, the cavity can be made perfectly planar, thus eliminating the major contributor to the gyroscope magnetic bias sensitivity.
One attempted technique for reducing the out-of-planeness errors caused by magnetic bias sensitivity is to modify the tolerance of the face angle errors of the gyroscope frame. Such attempt, however, has proven to be less than completely satisfactory in solving the magnetic bias sensitivity problem, since the errors resulting from the fabrication tolerances would still remain too large to be considered acceptable in practice.
Such attempted technique presents at least two potentially significant sources of error. The first source of error is the accuracy of the face angle measurement (about 0.5 arc-second). The second source of error is the uncertainty in the initial mirror alignment process. Additionally, there has been no relatively simple and inexpensive method to verify that a planar cavity has been achieved.
Theoretical and conceptual attempts have been made to calculate the derivation of the sensitivity of the square ring laser gyroscopes to magnetic fields and the relationship of this parameter to the cavity optical parameters such as out-of-planeness. The following results have been reached by the foregoing attempts. First, an elliptical polarization state must exist in the cavity in order for the magnetic field sensitivity to be present. In other words, a zero sensitivity corresponds to a purely linear polarization state.
The second result is that a perfectly planar beam path will yield a linear polarization state. In the general case where even a slight out-of-planeness exists, an elliptical polarization state is present. The degree of ellipticity varies continuously from zero, that is, in a linear polarization state, as out-of-planeness is introduced. The third result is that, if one of the two counter-propagating waves of the square ring laser gyroscope is purely linear in polarization, the other wave will also be linear in polarization.
One attempt to resolve the magnetic bias sensitivity utilizing the foregoing theoretical concept has been to determine the point at which a linear polarization state exists. This is done by first generating the passive cavity resonance using an external source laser wherein either the test cavity or the source are presumed to be tunable. The ratio of the p-polarization to the s-polarization component of the transmitted resonance signal is then measured. Finally, the p-polarization to the s-polarization ratio is then driven to a minimum value by moving the spherical mirrors in order to vary the out-of-planeness.
One of the most fundamental problems with such theoretical approach has been the cavity exit mirror which intervenes and which possesses non-negligible polarization properties. In fact, the following characteristics of the exit mirror act as a serious source of errors. First, the exit mirror acts as a polarization filter which transmits much more of the p-component relative to the s-component.
The second characteristic of the exit mirror is that the multi-layer dielectric mirror stack may possess a non-zero phase retardance between the s- and p-axes. The third and most important characteristic of the exit mirror is that the mirror substrate (which is generally made of material sold under the name Zerodur) has a non-zero birefringence due to the stresses in the material; that is, it changes ellipticity as the light beam passes therethrough.
The third characteristic is particularly important since it can, depending upon the orientation of the stress axis of the substrate material, introduce a sizeable ellipticity to a transmitted linearly polarized beam, thus invalidating the effectiveness of the foregoing theoretical attempt. The "ellipticity versus the out-of-planeness" curve would exhibit a minimum point, which will be displaced away from the zero out-of-planeness. Thus, the minimum measured out-of-planeness is not sufficiently accurate.